1. The Field of the Invention
This invention relates to pressure transducers for medical use. More particularly, the present invention is directed to a novel, disposable pressure transducer apparatus for use in the direct measurement and/or monitoring of intracompartmental pressure, such as intrauterine pressure during labor and childbirth.
2. The Prior Art
There are many situations in which intracompartmental pressure monitoring may be useful, including intraarticular, esophageal, intra-intestinal and intracranial. Perhaps one of the most common type of intracompartmental pressure monitoring is intrauterine monitoring.
Each year, approximately 3.5 million children are born within the United States. In order to assist physicians in treating a mother and child approaching childbirth, monitoring devices are quite commonly used during the final stages of labor to monitor both the mother's uterine contractions and the fetal heart rate. Such monitoring devices can quickly provide the physician with information about the rate, duration, and intensity of the mother's uterine contractions and the effect of the contractions on the fetal heart rate. This information can held the physician ensure that oxygen and nutrients are being properly transferred from the mother to the fetus during labor and childbirth and can help the physician identify potential problems before they become life-threatening.
Most frequently, uterine contractions and the fetal heart rate are monitored using devices which can be secured externally to the surface of the mother's abdomen. For example, a pressure sensitive button called a tocotransducer is often secured to the mother's abdomen to provide information about the frequency and duration of the uterine contractions. The fetal heart rate may then be monitored by an ultrasound transducer, a phonotransducer, or some other suitable device.
In most cases, externally secured monitoring devices can provide sufficient information to enable a physician to treat the mother and child during labor and childbirth. It will be appreciated, however, that the use of external monitoring devices may give rise to large measurement errors in some cases due to extraneous noise and/or movement by the mother. In approximately ten percent of the childbirths, where there is a significant risk of complications, a physician may wish to have more accurate measurements than can be obtained using external monitoring devices.
In order to obtain more reliable and accurate information about the mother's uterine contractions, a physician will often initiate intrauterine pressure monitoring. In addition to providing information about the rate and duration of the uterine contractions, intrauterine pressure monitoring can also provide information about the intensity of the uterine contractions. Importantly, since the uterine pressure is being measured directly, errors in measurement due to extraneous noise and movement by the mother are less likely than with external monitoring devices.
One of the most widely used techniques for intrauterine pressure measurement and monitoring uses a fluid-filled catheter inserted into the uterus and then connected externally to a pressure transducer. In using this technique, a rigid guide tube is inserted just inside the mother's cervix. A special catheter is then threaded through the guide tube until it extends into the uterus approximately 15 to 20 centimeters (cm). This catheter is filled with some type of solution, such as, for example, a sterile saline solution. Once the catheter is in place, the guide tube is removed from the cervix and slid away from the mother along the catheter.
After the in-dwelling catheter is positioned as described above, the other end of the catheter is fluid coupled to a pressure transducer. The pressure transducer is then connected to some type of monitor device near the patient's bedside. Typical monitor devices include cathode ray tube display devices, digital display and/or recording devices, printers, and plotters.
In addition to the proper set-up of the measurement equipment in the above-described manner, it is also important to prime the catheter with a sterile solution so that any air bubbles within the catheter are removed and a continuous fluid column is provided from the pressure transducer to the tip of the catheter within the uterus. Then, when the mother's uterus thereafter contracts, the increased intrauterine pressure displaces the fluid within the catheter, and the pressure transducer detects a change in the intrauterine pressure. The pressure transducer generates electrical signals representing the intrauterine pressure, and such signals are then amplified and displayed by the monitor device. Usually, the monitor device is used to display the mother's intrauterine pressure as a function of time, along with the fetal heart rate, and this data can then be used by the physician and other medical personnel to appropriately diagnose and treat the mother and child.
While the foregoing technique for intrauterine pressure monitoring is widely used and can produce reliable measurements, there are a number of significant difficulties associated with this technique. First, the necessity of using a rigid catheter guide tube to insert the catheter into the uterus can make catheter insertion somewhat awkward and difficult, as well as posing a potential threat of puncturing a wall of the uterus and causing hemmorrhage, or causing injury to the child. Moreover, since the distal end of the catheter is typically secured to a needle or some other coupling device, it is difficult to remove the rigid guide tube from the catheter after the catheter is inserted. Often, the rigid guide tube makes it awkward to move the in-dwelling catheter around once it is inside the uterus of the patient, and the catheter may occasionally break, bend or become removed as a result of being pinched by the guide tube.
Another disadvantage of the above-described technique is that the fluid column necessarily opens into the uterus. As a result of fluid displacement in the catheter, amniotic fluid from the uterus invariably enters the catheter and may interfere with accurate pressure monitoring. Sometimes, an air bubble will also enter the catheter, or uterine tissue may obstruct the open end of the catheter. In such cases, it is necessary to flush the catheter with sterile solution to remove the air bubble or obstruction, and it may occasionally be necessary to replace the catheter altogether.
A further difficulty with the foregoing technique is due to the difficulty of zero balancing the pressure transducer to insure that the static readings it produces are accurate. A transducer is balanced in order to establish atmospheric pressure at the baseline or zero point from which the patient's intrauterine pressure is referenced.
A transducer is often used with a disposable dome that fits over the transducer diaphragm. The dome has two ports, one on the side and one vertical. The side port is connected to the in-dwelling fluid-filled catheter after it is primed with sterile solution. The other port is generally used for balancing and calibration.
In order to balance the transducer, the vertical port of the dome is opened to atmosphere and the other port is open to the catheter into the patient. The transducer is then raised or lowered until the top of the vertical port is level with the position of the in-dwelling tip of the fluid-filled catheter. For each inch off the proper level, there will be an error in the pressure reading of about 2 millimeters (mm) of mercury (Hg). The monitor is then zeroed and the transducer port recapped.
The difficulty in accurately balancing the transducer using the above-described technique for intrauterine pressure monitoring will be readily appreciated, since the tip of a catheter inside the mother's uterus cannot be seen. Consequently, the location of the tip of the catheter can only be estimated, and it is virtually impossible to determine whether the transducer is producing totally accurate pressure readings. One further disadvantage of the above-described system is that there are potential errors in the transducer readings that may be introduced as a result of overlying compliant tubing, or failure to adequately flush all air from the system prior to use. This will result in overdamping of the pressure readings.
It will be further appreciated that one of the most important components of an intracompartmental pressure monitoring system is the pressure transducer. Significantly, the accuracy and reliability of the pressure transducer set an upper limit to the quality of the pressure data which can be obtained. Therefore, those skilled in the art of pressure monitoring have attempted to develop pressure transducers which have a high degree of reliability, sensitivity, and accuracy.
Pressure transducers have typically been quite expensive to manufacture. Consequently, these transducers have often been provided in the form of a reusable instrument which can be connected to a fluid-filled catheter outside the patient by means of a disposable dome. Although such reusable pressure transducers can produce acceptable results, the use of such transducers has a number of significant disadvantages. First, since the transducer is to be used by a number of patients, the catheter and transducer dome must be sterilized for each use; and this sterilization procedure can be both time consuming and expensive. Further, the transducer must undergo periodic maintenance in order to assure its proper functioning and operation. All of these factors have made the use of reusable pressure transducers somewhat burdensome and inconvenient.
With the growth of the semiconductor industry and the recent developments in integrated circuit technology, disposable pressure transducers are increasingly used in place of conventional re-usable transducers. Nevertheless, these disposable pressure transducers also suffer from a number of disadvantages.
In order for a transducer to function properly, the diaphragm of the transducer must be vented on one side to a substantially constant pressure. Typically, one side of the diaphragm of the transducer is vented to atmospheric pressure. However, in intracompartmental applications such as intrauterine monitoring, when a transducer is positioned within the uterus or other body compartment, it can be extremely difficult to properly vent the transducer, and the lack of proper venting can make it very difficult to obtain accurate pressure measurements.